Ensure all exported paths use Posix style separators.

mbed_config.h is a file that is placed in your project root when you
export. This allows the configuration to be used by your IDE. When this
file already exists (due to a previous export), it changed the compiler
include option on Windows from "-include mbed_config.h" to "-include
.\mbed_config.h". The "\" character was interpreted as an escape
character "\m" and broke the build. This converts all resource paths to
use the Posix path separator "/" before writing project files to avoid
this issue.

Author: bridadan

tools/export/exporters.py

@@ -92,6 +92,7 @@ def __init__(self, target, export_dir, project_name, toolchain,
         self.toolchain = toolchain
         jinja_loader = FileSystemLoader(os.path.dirname(os.path.abspath(__file__)))
         self.jinja_environment = Environment(loader=jinja_loader)
+        resources.win_to_unix()
         self.resources = resources
         self.generated_files = []
         self.static_files = (